Thermal and Water Homeostasis

The thermal requirements of amphibians vary tremendously between species and even between geo- graphic variants of the same species. An understanding of a species's thermal preferences and tolerances is essential for guiding vivarium design and husbandry efforts, for many of an amphibian's metabolic processes are impacted by the temperature at which

they occur (Hutchison & Dupre, 1992; Rome et aI., 1992). (See Chapter 5, Amphibian Husbandry and Housing.) If an amphibian cannot maintain its body temperature appropriately it will fail to thrive in captivity.

If an amphibian is maintained at temperatures below its preferred body temperature, it may show signs such as inappetence, lethargy, abdominal bloating from decomposition of ingesta, poor growth rate, and immunosuppression. When maintained at temperatures above its preferred body temperature, an amphibian may show signs such as agitation, excessive movement, changes in skin color, inappetence, weight loss despite good appetite, and immunosuppression. An appropriately constructed vivarium should provide a mosaic of temperatures above and below the preferred body temperature for the species housed so that the inhabitants can thermoregulate.

Some species, such as the White's treefrog, Pelodryas caerulea, show a tremendous ability to withstand elevated body temperatures, an adaptation which may serve to conserve water. Individual White's treefrogs, Pelodryas caerulea, may bask to achieve a body temperature in excess of 38°C (100°F). Metamorphosing amphibians may seek out temperatures different from the larvae or adults, as may ill amphibians.

Behavioral fever in amphibians has been demonstrated (Stebbins

& Cohen, 1995), but the fever's potential impact on successful treatment of disease in amphibians has been ignored. Given the limitations of current understanding of the thermal requirements of amphibians, it is important for the enclosures used to provide a mosaic of thermal environments above, below, and including the known or presumed preferred body temperature for the species in question.

Amphibians have developed a wide variety of behavioral and physiological adaptations to cope with the challenges of existing in either an aquatic (Boutilier et aI., 1992) or terrestrial (Shoemaker et al. 1992) environment.

The skin of most amphibians, with the exception of a few species of treefrog (e.g., leaf frogs, Phyllomedusa spp., reedfrogs, Hyperolius spp., foam-nest treefrogs, Chiromantis spp.), is a negligible barrier to water loss. The rates of water loss by evaporation are much higher for amphibians than other terrestrial vertebrates, a fact that serves as a limiting factor to the range and activity pattern of amphibian species.

Many terrestrial amphibians, especially those lungless forms (e.g., plethodontid salamanders), must remain moist in order for gas exchange to be effective. Thus many amphibians limit their activity patterns to exploit periods of elevated humidity (e.g., during or immediately following rain, during fog, or at night).

Several treefrog species possess a skin that is extremely resistant to evaporative water loss. The underlying etiology of this phenomenon is only well documented in the genera Phyllomedusa (Blaylock et aI., 1976). Members of this genus have lipid glands in their skin that secrete a waterproof substance composed of waxy esters and other fatty acid compounds (McClanahan et aI., 1978). The secretions of these glands are smeared over the surface of the frog with stereotyped movements of its feet. This lipid coating imparts a surface resistant to cutaneous water loss that is comparable to that of many reptiles.

The lipid glands possessed by phyllomedusine frogs are lacking in other species of hylid frogs thus far studied. In other amphibians a combination of several features of the dermis and epidermis may contribute to the waterproofing of the skin. Structures that have been described that may reduce evaporative water loss through the skin include stacked iridophores, a band of undetermined material immediately beneath the stratum corneum, or dried mucus on the surface of the skin. Doubtless other water reduction mechanisms await elucidation.

It is notable that the above described anatomic and physiologic adaptations to reduce evaporative water loss are lacking on the ventral surface of many amphibians. The ventral surface is an important route for the uptake of water from the environment, and in anurans a modified area of the pelvic ventrum is some-times known as a "drinking patch" (Parsons, 1994). The more adapted an amphibian is to a xeric environment, the more effective it is at extracting moisture from the soil. The drinking patch accounts for up to 80% of the water uptake of an anuran.

Little water uptake occurs through the gastrointestinal tract except in some species such as the waxy treefrog, Phyllamedusa sauvagii (McClanahan & Shoemaker, 1987). Most amphibians cannot be said to drink in the manner of other terrestrial vertebrates, thus oral fluids are of little help in combatting dehydration in the amphibian. Soaking the dehydrated amphibian in shallow water and, on occasion, subcutaneous or in-tracoelomic administration of appropriately dilute dextrose/electrolyte solutions are the suggested methods of combatting dehydration.

Certain behaviors minimize water loss to the environment. Hylid treefrogs have a water conserving posture which reduces the surface area exposed to evaporative effects of the environment. The limbs are adducted and the head is pressed against the surface. The ventrum is well protected by the rest of the anuran's body. If a treefrog is consistently in this posture, it suggests that the humidity of the enclosure is too low, and appropriate corrective measures should be undertaken.

The osmolality of a well hydrated amphibian is over 200 mOsm (Bentley, 1971). To increase the water absorbability, some amphibians may selectively retain additional solutes so that it can extract water from much drier soils. Since the amphibian kidney cannot concentrate urine above the osmolality of plasma, one of the main methods of water conservation is the ability to tolerate a wide fluctuation in the osmolality and composition of its plasma. This physiological adaptation can confound the interpretation of a single plasma biochemistry in the amphibian, thus multiple biochemistries, sufficient to assess plasma osmolality, are recommended as part of the amphibian diagnostic evaluation.

Aquatic amphibians are under a different water flux than terrestrial species, as they are immersed in a hyposmotic environment. Aquatic amphibians are adapted to excreting excess water while conserving plasma solutes. Water is continually being absorbed through the skin and gills, if present. If the excretory function of the kidneys fail, or the cutaneous ex-change functions fail, the plasma will rapidly dilute with absorbed water while ions and other solutes are lost. This is problematic, for it is difficult to initiate any corrective action (i.e., reestablishing "normal" plasma osmolality via parenteral dextrose/electrolyte solutions or colloid solutions) that does not exacerbate the problem. (See Section 24.9, Maintaining the Electrolyte Balance of the IIIAmphibian.) Volume overload through expansion of the blood (plasma) volume eventually places undue stress on the heart and quickly incapacitates the amphibian.

Aquatic amphibians generally excrete ammonia as their main nitrogenous waste, and this is excreted not only through the kidney but also by the skin and gills, if present. Since ammonia is so toxic, this is not an option for amphibians that aren't surrounded by water at all times. Terrestrial amphibians convert a large portion of their nitrogenous wastes to urea via ornithine cycle enzymes located within hepatocytes.

Urea is less cytotoxic than ammonia and can be stored inside the bladder without consequence. When the amphibian has access to water, it will void its urea-laden urine. Some species can actually switch back and forth between ammonia as the main nitrogenous waste and the production of urea, depending on the availability of water (Balinsky et al., 1961). A few species of anurans (e.g., phyllomedusine frogs, rhacophorid foamnest frogs) are known to produce uric acid as a further water conservation method (Loveridge, 1970, Shoemaker et al., 1972).

A common pet anuran, waxy treefrog, Phyllamedusa sauvagii, is uricotelic. Amphibians appear to have a different localization of their purine cycle enzymes within the hepatocyte than occurs in other uricotelic vertebrates (i.e., nonavian and avian reptiles), and the level of activity appears slightly lower in amphibians (Smith & Campbell, 1988). A final alternative to nitrogen storage prior to elimination is the incorporation of nitrogen into purines within iridiophores, which occurs in some reed frogs, Hyperalius spp., when experiencing dehydration (Geise & Linsenmair, 1986).

Some terrestrial amphibians can tolerate a loss of body water up to 40% of their body weight, and can maintain constant plasma solute concentration until the reservoir of water in the bladder is expended (Shoemaker et aI., 1992). Under conditions of restricted water, amphibians cease to produce urine as conservation of body water takes precedence over excretion of nitrogenous wastes. Thus, dehydrated amphibians are often suffering from varying levels of ammonia or even urea intoxication following rehydration. Systemic gout as a result of dehydration is possible although undocumented. Urate bladder stones are a more common sequelae to dehydration and have been discovered in several waxy treefrogs, Phyllomedusa sauvagii.

Given the permeability of amphibian skin, many exogenous compounds are freely absorbed. This absorption is useful since it allows many beneficial substances such as antibiotics and anthelmintics to be administered topically. Toxic substances, such as chlorine from chlorinated water, may also readily be absorbed by an amphibian, making them potentially more sensitive to many environmental contaminants.